Magnets and the concept of magnetism have been known since ancient times. Faraday, in the 1830's, demonstrated the relationship between an electric charge and magnetism. When an electric charge moves, it is said to constitute an electric current. When an electric current flows, it generates a magnetic field in the space around it just as if the current system had been replaced by a magnet system with a particular shape. It takes a force, called an electromotive force (EMF), analogous to the pressure needed to cause water to flow in a pipe to make a charge move and, thus, produce an electric current. Faraday established that when an object capable of conducting electric current was moved through a magnetic field an EMF was set up in the conductor capable of producing electric current. He also demonstrated that when the magnetic field which "threaded" a conducting object was changed, an EMF was also produced. Accordingly, electricity produces magnetism and magnetism produces electricity. Faraday also recognized the concept that magnetism exists only in a closed loop. It can be shown that the driving force, called the magnetomotive force (MMF), in a magnetic circuit of an electromagnet, analogous to EMF in an electric circuit, is proportional both to the number of turns which form the coil and the current in that coil. A recognition of the relationship between magnetism and electricity has permitted the formation of electromagnets of varying strength and capacity by winding coils onto a core material.
The advent of new technologies utilizing magnets has increased the need for magnets having greater field uniformity and stability. For example, Nuclear Magnetic Resonance, normally referred to as NMR, a technique discovered in 1945 for measuring the magnetic properties of individual atomic nuclei has become widely used in chemical analysis to determine structures of molecules. Although NMR technology was originally used primarily as a tool to the chemist for determining the structure of new compounds, particularly in the pharmaceutical industry and in university research laboratories, the technique is now being applied to a much broader spectrum, including to the study of biological problems and in medical research such as in the detection of cancer in tissue. In an NMR system it is essential to have magnets which have a magnetic field of high uniformity and known and well-defined pattern. By having a magnetic field of high uniformity and known and well-defined pattern, it is possible to pick up signals from the atomic nuclei being acted upon, and then have that signal analyzed and recorded.
Electron Magnetic Resonance (EMR) has also provided a need for improved magnets. The term Magnetic Resonance (MR) embraces both NMR and EMR.
In the prior art fabrication of the magnets for MR application, precision wound coils are produced under the guidance of very slow computer controlled coil winders. After fabrication, the coils are carefully tweaked and adjusted to maximize uniformity. In operation they are accompanied by a relay rack full of electronics requiring constant attention to maintain field uniformity. Furthermore, the only fields allowing theoretical description are free space fields, and as a result the current designs all use fields that extend far beyond the magnet system. The fields external to the system are disturbed by magnetic objects. Movement of such objects contributes to the frequent need for readjustment.
Accordingly, there is a need for magnets having a magnetic field of high uniformity and stability for use in MR and the like systems which are relatively inexpensive and which can be more rapidly fabricated.